Broad band coaxial coupling for travelling wave tubes



July 29, 1958 w. KLEIN 2,845,570 \BROAD BAND COAXIAL COUPLING FOR TRAVELLING WAVE TUBES Filed April 7, 1953 INVENTOR WERNER KLE N ATTORNEY BROAD BAND coh xixt COUPLING FOR TRAYELLING WAVEJTUBES Werner Klein, Komtal, Stuttgart, Germany, assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware The present invention relates to travelling wave tubes and particularly to the input and output wave coupling means.

Travelling wave tubes consist of an electron gun system for producing an electron beam, a wave retarding line and an electron collector for collecting the electrons of the beam. A helix serves in most cases as the retarding line. Coupling-in and coupling-out of the wave energy to the ends of the helical line is in many cases capacitive. The ends of the line in such cases are not brought outside the tube. There are however other forms in which ends of the helical line are brought out. For example, in order to couple the helical line to a coaxial connecting line, the extended end of the helix may be connected to the inner conductor of the coaxial line. The travelling wave tube is positioned within a cylindrical conductive housing which serves as the outer conductor of the coaxial line of which the helix is the inner conductor. For convenience of reference such a structure having a helical center conductor is referred to as a helical coaxial line, to distinguish over other forms of coaxial lines.

In the case of tubes with coaxial coupling, matching of the connecting coaxial lines to the helical coaxial line is obtained for example by sliders that slide upon the outside of the glass bulb surrounding the helical conductor and fit within the cylindrical housing surrounding the tube. Such arrangements produce satisfactory matching only up to frequencies of approximately 2000 mc. if sutficiently large band-width is to be obtained. The same applies to cases of capacitive coupling of the inner conductor of coaxial lines to the helix.

An object of the present invention is the provision of a broad band coaxial coupling for travelling wave tubes, also such a coupling for higher frequencies up to about 4000 mc. and higher, with satisfactory matching.

In accordance with the main feature of the present invention, two transformation members are employed at each end of the tube, that is, two transformation members are used for the input coupling and two transformation members are used for the output coupling tothe tube. Employment of both transformation members at each end provides a band-width of greater than at the known frequencies. The mis-matching factor m is in this range greater than 0.8. The transformation members are tuned to produce maximum coupling at those frequencies that serve at any given time as the mean frequencies of the band.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood, by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein the single figure is a schematic diagram of a portion of a travelling wave tube at one end of the helix showing the two transformation members thereat. It is to be understood that two l 2 45 5 9 Eateries! Ju y29 1 .58

similar transformation embers are also arranged at the other endo f'the 'helix.

Referring no'w to theffigure, the he fisr sq a h s a las is lsssa a 7mi? h'i nd f'ihe he W ttss mias man is provided surrounding a portion of theglassbplb 5 within which the helix l is mounted. This transforming member 3 is Wide enough to extend over and cover at least 5 turns of the helix 1. The transformation member 4 makes galvanic contact with the outer conductor of the helical line 3. The transformation member makes a capacitive coupling to the helix and, with the aid of a slotted cylinder, establishes a galvanical connection with the coaxial exterior conductor, the transformation member thereby acting as a quarter wave-length transformer between the coaxial helical line and the end of the helix. The transformation member exhibits an axial length of about one uneven multiple of a quarter wave-length, whereby at least five turns of the helix are covered, when the pitch of the helix is chosen to include about four turns per wave-length.

The second transformation member lies within the coaxial coupling line 6 whose exterior conductor 7 is galvanically connected and may be integrally made with the outer conductor 2 of the helix coaxial line 3. The extended end 8 of the helix 1 is galvanically connected to an end portion 9 of the inner conductor 10 of coaxial line 6. End portion 9 is of a diameter about half as large as the diameter of the remainder 11 of the inner conductor 10. A metallic sleeve 12 fitted partly over end portion 9 and partly over the remainder 11 of the inner conductor, allows changing the effective length of said end portion 9 in order to achieve the most favorable point for the transformation. The length of the sleeve 32 depends on the selected mean frequencies. The sleeve is made axially slidable by means of a dielectric support 13. This dielectric support 13 represents an additional capacity, together with the sleeve 12, terminating the coaxial line, and establishing the transformation with the end of the helical line.

In avoiding feedback within the tube, for example, within the helix or within the helical coaxial line, it is proposed that an attenuating substance 14 be inserted in the middle of the tube between the outer conductor 2 and the glass tube 5. This causes but little loss of the helix in absence of the electron beams but permits reaching stable operation of the tube with a matching to the helix of approximately 0.8.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forthin the objects thereof and in the accompanying claims.

What is claimed is:

1. In an electromagnetic wave structure, a travelling wave tube having an envelope and a conductive helix Within said envelope, an outer conductor outside and spaced from said envelope surrounding said helix and forming therewith a first coaxial line, a connecting coaxial line having an outer conductor, connected to the outer conductor of said first coaxial line adjacent one end of the helix, a first impedance transforming member including a capacitive plunger slidably mounted between the outside of the tube envelope and the inside of the outer conductor of said first coaxial line and surrounding a number of turns of the helix towards said one end thereof and a second impedance transforming member towards said one end of the helix including an end portion of the inner conductor of the connecting coaxial line of a diameter substantially less than the diameter of the remainder of said inner conductor of which it is a part,

helix 1' form swith prises a metal collar slidably mounted over said end por-' tion and connected to the remainder of the same inner conductor;

3. A structure according to claim 1 further including similar first and second impedance transforming members at the opposite end of said helix.

References Cited in the file of this patent UNITED STATES PATENTS Lindenblad Apr. 21, 1953 Nergaard et a1 July 13, 1948 Dohler et a1. Dec. 25, 1951 Kleen et al Oct. 27, 1953 Harman "I.-- Mar. 16, 1954 FOREIGN PATENTS Great Britain Sept. 17, 1952 

